1. Field of the Invention
The present invention relates to a rotary deflector, an optical scanning unit and an image forming apparatus for use in a laser printer, a digital copier and the like.
2. Related Art
An optical scanning unit is provided with a driving motor for rotating a polygon mirror at high speed. However, it is unable to realize a high image quality because optical parts vibrate, scanning position deviates periodically and nonuniformity of pitch occurs on an image unless vibration caused by the driving motor is reduced. Still more, noise may be generated when a housing of the optical scanning unit resonates with a base of the image forming apparatus.
In order to eliminate such a problem, Japanese Patent Laid-Open No. Hei. 5-264916 has disclosed a method of controlling the resonating condition of the housing by changing fixing points of the base and the housing of the image forming apparatus per type of machine or by increasing/reducing a mass element to be added to the housing, to avoid the resonance frequency of the housing in correspondence to changes of vibrating frequency of a source of vibration.
However, there has been a problem that when the fixing points of the housing are changed, the position of a laser beam fluctuates and the image quality degrades because the amount and the mode of deformation of the housing change.
The driving motor for rotating the polygon mirror is arranged so as to switch the rotational speed during the standby state and the image forming time to reduce power consumption and to switch the rotational speed corresponding to an image density in order to accommodate to plural image densities (resolutions).
However, because the above-mentioned prior arts have had no arrangement of reducing the vibration of the vibrating source itself, there has been a problem that when the rotational speed is changed, the vibrating frequency of the driving motor approaches to the vibrating frequency of the housing, thus resonating and causing noise.